As examples of a radio communication apparatus used in an automobile, there are a hands-free apparatus wirelessly connected to a mobile phone by Bluetooth (registered trademark), a music player, and the like. In particular, for example, like a car navigation apparatus and an in-vehicle audio apparatus, in-vehicle apparatuses incorporating functions of the hands-free apparatus and the music player apparatus are spread (see Non Patent Literature 1).
Further, there is an increasing demand for incorporating a wireless connection function by a wireless LAN (Local Area Network) in an in-vehicle apparatus, wirelessly connecting an apparatus having a wireless LAN communication function, which is carried into a vehicle by a user, and the in-vehicle apparatus, and performing radio communication among a plurality of apparatuses even in the vehicle. The wireless LAN is a radio communication technology that makes use of IEEE (The Institute of Electrical and Electronics Engineers) 802.11a/b/g/h/ac standardized by international standardization activities or Wi-Fi (registered trademark) (see Non Patent Literature 2). In particular, as communication forms by the wireless LAN, a lot of uses for communication between devices, such as personal computers, handheld game machines, and smart phones, are becoming widespread in public places, businesses, homes, and the like.
A radio communication system using the wireless LAN has been developed on the premise that the radio communication system is used in indoor and outdoor stationary environments. If the wireless communication system is introduced, it is possible to provide subscribers or users with an Internet connection service by radio in a railroad station, a store, and the like and perform communication anywhere in a home. In particular, in the case of the home, introduction of a radio communication relay station such as a wireless LAN router tends to increase according to the spread of personal computers, handheld game machines, smart phones, and the like equipped with the wireless LAN communication function.
When the number of the apparatuses using the wireless LAN increases as described above, interference with other radio communication networks poses a problem. In the wireless LAN, a 2.4-GHz band or a 5-GHz band is used. In the Bluetooth, the 2.4-GHz band is also used. Therefore, Bluetooth communication could be a cause of interference. In a stationary environment like the home, in order to avoid interference with wireless LAN networks of neighbors within the coverage of a radio wave, a method is adopted in which a radio access point (router) checks channels in use during a start and selects a non-interfering channel. Because of the stationary environment, if a channel is once secured, it is less likely that interference occurs with the respective wireless communication networks in the neighborhood. Therefore, it is possible to continue stable communication for a long period. Patent Literatures 1 and 2 disclose a method of setting, taking into account channel qualities of the Bluetooth and the wireless LAN, an FH (frequency hopping) channel of the Bluetooth such that communications do not interfere with each other.
However, when the wireless LAN is used under a moving environment such as the inside of an automobile, it is highly likely that interference is caused depending on a moving destination of the automobile. For example, in a state in which the automobile is parked on a road in a residential area or a state in which the automobile is moving extremely slowly in a residential area, when the channels of the radio wave of radio communication in the automobile and the radio wave of radio communication used in a home within the coverage of the radio wave coincide with each other or when the channel widths overlap, both or one of the radio communications is affected by interference. The same problem tends to occur when the automobile is approaching a building or the like where the radio communication is used not only in the residential area but also in a business district, a shopping district, and the like. Patent Literature 3 discloses an interference avoidance technology in the 2.4-GHz band widely used in general. According to the description of Patent Literature 3, concerning other wireless LANs, for example, information concerning other wireless LAN stations that could be interference is collected at a timing determined on the basis of the GPS function and position information by using a scan function of the wireless LAN installed in an own automobile. When it is likely that interference occurs between the wireless LAN and other wireless LAN stations, a wireless LAN channel in use is changed.
In a limited space such as the inside of an automobile, a mutual interference could occur when the wireless LAN and the Bluetooth that use the 2.4-GHz band are concurrently used. In that case, by using the wireless LAN that uses the 5-GHz band, for example, IEEE802.11a/n/ac, it is possible to avoid the interference with the Bluetooth operating in the 2.4-GHz band.
As a system anticipated to interfere with the wireless LAN, there is a radar system that uses the 5-GHz band. For example, a meteorological radar apparatus emits a radio wave from an antenna and receives the radio wave reflected by rain, cloud, and the like to observe the states of the rain, the cloud, and the like. Therefore, the meteorological radar apparatus includes a receiver having extremely high sensitivity. Similarly, an aviation radar, a military radar, and the like also include receivers having extremely high sensitivity. When a communication system such as a wireless LAN system uses a communication channel including frequencies of radar waves emitted by the above radars, interference with the radar apparatuses, in particular, the receivers may occur.
For example, in the wireless LAN in the 5-GHz band, interference with the above radars is a problem. To secure interoperability, it is necessary to have a radar detection function called DFS (Dynamic Frequency Selection) or TPC (Transmitter Power Control) and a channel shift function (or a service stop function) during detection. Specifically, when, among frequency bands called W52 (5.15 to 5.25 GHz), W53 (5.25 to 5.35 GHz), and W56 (5.47 to 5.725 GHz), W53 and W56 are used, it is essential to have the DFS function and the TPC function.
In the conventional DFS, as pre-service monitoring (Channel Availability Check), presence or absence of a radar wave is checked by monitoring a channel for sixty seconds without performing transmission before network establishment. During a service, as in-service monitoring, a radar wave is continuously monitored while communication is performed. If the radar wave is detected, within a channel move time (ten seconds or less), transmission in the channel is completely stopped (the communication is stopped) or the channel is switched to another channel without interference and the communication is continued. Note that communication cannot be performed for thirty minutes or more in the channel in which the radar wave is detected. A radio LAN system that uses the 5-GHz band having a plurality of support communication channels searches for another frequency channel without interference and moves. Therefore, time is required for the search.
Therefore, when large-capacity data such as video data is transmitted on a real-time basis using the radio communication system equipped with the wireless LAN in the 5-GHz band, if a communication channel is changed during the transmission of the video data, deficiencies occur, for example, the transmission of the video data is interrupted during the change, the video is disturbed on the receiver side of the video data, and the video is stopped. As measures against this problem, a method has been examined that realizes video transmission without the deficiencies such as the disturbance of the video and the stop of the video by selecting beforehand a communication channel that does not cause interference with a radar apparatus in order to eliminate the need for change of a communication channel during communication, (see, for example, Patent Literature 4).